This invention relates to an OFDM receiving method and an OFDM receiving apparatus. More particularly, the invention relates to an OFDM receiving method and OFDM receiving apparatus for receiving a signal multiplexed according to Orthogonal Frequency Division Multiplexing (OFDM) and applying FFT processing to the receive signal to demodulate transmit data.
Multicarrier modulation schemes have become the focus of attention as next-generation mobile communication schemes. Using multicarrier modulation not only makes it possible to implement wideband, high-speed data transmission but also enables the effects of frequency-selective fading to be mitigated by narrowing the band of each subcarrier. Further, using orthogonal frequency division multiplexing not only makes it possible to raise the efficiency of frequency utilization but also enables the effects of inter-symbol interference to be eliminated by providing a guard interval for every OFDM symbol.
(a) of FIG. 12 is a diagram useful in describing a multicarrier transmission scheme. A serial/parallel converter 1 converts serial data to parallel data and inputs the parallel data to orthogonal modulators 3a to 3d via low-pass filters 2a to 2d, respectively. In the Figure, the conversion is to parallel data comprising four symbols. Each symbol includes an in-phase component and a quadrature component. The orthogonal modulators 3a to 3d subject each of the symbols to orthogonal modulation by subcarriers having frequencies f1 to f4 illustrated in (b) of FIG. 12, a combiner 4 combines the orthogonally modulated signals and a transmitter (not shown) up-converts the combined signal to a high-frequency signal and then transmits the high-frequency signal. With the multicarrier transmission scheme, the frequencies are arranged, as shown at (b), in such a manner that the spectrums will not overlap in order to satisfy the orthogonality of the subcarriers.
In orthogonal frequency division multiplexing, frequency spacing is arranged so as to null the correlation between a modulation band signal transmitted by an nth subcarrier of a multicarrier transmission and a modulation band signal transmitted by an (n+1)th subcarrier. (a) of FIG. 13 is a diagram of the structure of a transmitting apparatus that relies upon the orthogonal frequency division multiplexing scheme. A serial/parallel converter 5 converts serial data to parallel data comprising a plurality of symbols (I+jQ, which is a complex number). An IDFT (Inverse Discrete Fourier Transform) 6, which is for the purpose of transmitting the symbols as subcarriers having a frequency spacing shown in (b) of FIG. 13, applies an inverse discrete Fourier transform to the frequency data to effect a conversion to time data, and inputs the real and imaginary parts to an orthogonal modulator 8 through low-pass filters 7a, 7b. The orthogonal modulator 8 subjects the input data to orthogonal modulation, and a transmitter (not shown) up-converts the modulated signal to a high-frequency signal. In accordance with orthogonal frequency division multiplexing, a frequency placement of the kind shown in (b) of FIG. 13 becomes possible, thereby enabling an improvement in the efficiency with which frequency is utilized.
In recent years, there has been extensive research in multicarrier CDMA schemes (MC-CDMA) and application thereof to next-generation wideband mobile communications is being studied. With MC-CDMA, partitioning into a plurality of subcarriers is achieved by serial-to-parallel conversion of transmit data and spreading of orthogonal codes in the frequency domain. Owing to frequency-selective fading, subcarriers distanced by their frequency spacing experience independent fading on an individual basis. Accordingly, by causing code-spread subcarrier signals to be distributed along the frequency axis by frequency interleaving, a despread signal can acquire frequency-diversity gain.
An orthogonal frequency/code division multiple access (OFDM/CDMA) scheme, which is a combination of OFDM and MC-CDMA, also is being studied. This is a scheme in which a signal, which has been divided into subcarriers by MC-CDMA, is subjected to orthogonal frequency multiplexing to raise the efficiency of frequency utilization.
A CDMA (Code Division Multiple Access) scheme multiplies transmit data having a bit cycle Ts by spreading codes C1 to CN of chip frequency Tc using a multiplier 9, as shown in FIG. 14, modulates the result of multiplication and transmits the modulated signal. Owing to such multiplication, a 2/Ts narrow-band signal NM can be spread-spectrum modulated to a 2/Tc wideband signal DS and transmitted, as shown in FIG. 15. Here Ts/Tc is the spreading ratio and, in the illustrated example, is the code length N of the spreading code. In accordance with this CDMA transmission scheme, an advantage acquired is that an interference signal can be reduced to 1/N.
According to the principles of multicarrier CDMA, N-number of items of copy data are created from a single item of transmit data D, as shown in FIG. 16, the items of copy data are multiplied individually by respective ones of codes C1 to CN, which are spreading codes (orthogonal codes), using multipliers 91 to 9N, respectively, and products DC1 to DCN undergo multicarrier transmission by N-number of subcarriers of frequencies f1 to fN illustrated in (a) of FIG. 17. The foregoing relates to a case where a single item of symbol data undergoes multicarrier transmission. In actuality, however, as will be described later, transmit data is converted to parallel data of M symbols, the M-number of symbols are subjected to the processing shown in FIG. 16, and all results of M×N multiplications undergo multicarrier transmission using M×N subcarriers of frequencies f1 to fNM. Further, orthogonal frequency/code division multiple access can be achieved by using subcarriers having the frequency placement shown in (b) of FIG. 17.
FIG. 18 is a diagram illustrating the structure on the transmitting side of MC-CDMA (the structure of a base station). A data modulator 11 modulates transmit data of a user and converts it to a complex baseband signal (symbol) having an in-phase component and a quadrature component. A time multiplexer 12 time-multiplexes the pilot of the complex symbol to the front of the transmit data. A serial/parallel converter 13 converts the input data to parallel data of M symbols, and each symbol is input to a spreader 14 upon being branched into N paths. The spreader 14 has M-number of multipliers 141 to 14M. The multipliers 141 to 14M multiply the branched symbols individually by codes C1, C2, . . . , CN constituting orthogonal codes and output the resulting signals. As a result, subcarrier signals S1 to SMN for multicarrier transmission by N×M subcarriers are output from the spreader 14. That is, the spreader 14 multiplies the symbols of every parallel sequence by the orthogonal codes, thereby performing spreading in the frequency direction. Though codes (Walsh codes) C1, C2, . . . , CN that differ for every user are indicated as the orthogonal codes used in spreading, in actuality channel identification codes (Gold codes) G1 to GMN are further multiplied by the signals S1 to SMN.
A code multiplexer 15 code-multiplexes the subcarrier signals generated as set forth above and the subcarriers of other users generated through a similar method. That is, for every subcarrier, the code multiplexer 15 combines the subcarrier signals of a plurality of users conforming to the subcarriers and outputs the result. A frequency interleaver 16 rearranges the code-multiplexed subcarriers by frequency interleaving, thereby distributing the subcarrier signals along the frequency axis, in order to obtain frequency-diversity gain. An IFFT (Inverse Fast Fourier Transform) unit 17 applies an IFT (Inverse Fourier Transform) to the subcarrier signals that enter in parallel, thereby effecting a conversion to an OFDM signal (a real-part signal and an imaginary-part signal) on the time axis. A guard-interval insertion unit 18 inserts a guard interval into the OFDM signal, an orthogonal modulator applies orthogonal modulation to the OFDM signal into which the guard interval has been inserted, and a radio transmitter 20 up-converts the signal to a radio frequency, applies high-frequency amplification and transmits the resulting signal from an antenna.
The total number of subcarriers is (spreading ratio N)×(number M of parallel sequences). Further, since fading that differs from subcarrier to subcarrier is sustained on the propagation path, a pilot is time-multiplexed onto all subcarriers and it is so arranged that fading compensation can be performed subcarrier by subcarrier on the receiving side. The time-multiplexed pilot is a common pilot that all users employ in channel estimation.
FIG. 19 is a diagram useful in describing a serial-to-parallel conversion. Here a common pilot P has been time-multiplexed to the front of one frame of transmit data. It should be noted that the common pilot P can be dispersed within a frame, as will be described later. If the common pilot per frame is, e.g., 4×M symbols and the transmit data is 28×M symbols, then M symbols of the pilot will be output from the serial/parallel converter 13 as parallel data the first four times, and thereafter M symbols of the transmit data will be output from the serial/parallel converter 13 as parallel data 28 times. As a result, in the period of one frame the pilot can be time-multiplexed into all subcarriers and transmitted. By using this pilot on the receiving side, channel estimation is performed on a per-subcarrier basis and channel compensation (fading compensation) becomes possible.
FIG. 20 is a diagram useful in describing insertion of a guard interval. If an IFFT output signal conforming to M×N subcarrier samples (=1 OFDM symbol) is taken as one unit, then guard-interval insertion signifies copying the tail-end portion of this symbol to the leading-end portion thereof. Inserting a guard interval GI makes it possible to eliminate the effects of inter-symbol interference ascribable to multipath.
FIG. 21 is a diagram showing structure on the receiving side of MC-CDMA. A radio receiver 21 subjects a received multicarrier signal to frequency conversion processing, and an orthogonal demodulator subjects the receive signal to orthogonal demodulation processing. A timing-synchronization/guard-interval removal unit 23 establishes receive-signal timing synchronization, removes the guard interval GI from the receive signal and inputs the result to an FFT (Fast Fourier Transform) unit 24. The FFT unit 24 executes FFT processing and converts the signal in the time domain to N×M-number of subcarrier signals (subcarrier samples) at an FFT window timing. A frequency deinterleaver 25 rearranges the subcarrier signals in an order opposite that on the transmitting side and outputs the signals in the order of the subcarrier frequencies.
After deinterleaving is carried out, a fading compensator 26 performs channel estimation on a per-subcarrier basis using the pilot time-multiplexed on the transmitting side and applies fading compensation. In the Figure, a channel estimation unit 26a1 is illustrated only in regard to one subcarrier. However, such a channel estimation unit is provided for every subcarrier. That is, the channel estimation unit 26a1 estimates the influence exp(jφ) of fading on phase using the pilot signal, and a multiplier 26b1 multiplies the subcarrier signal of the transmit symbol by exp(−jφ) to compensate for fading.
A despreader 27 has M-number of multipliers 271 to 27M. The multiplier 271 multiplies N-number of subcarriers individually by codes C1, C2, . . . , CN constituting orthogonal codes (Walsh codes) assigned to users and outputs the results. The other multipliers also execute similar processing. As a result, the fading-compensated signals are despread by spreading codes assigned to each of the users, and signals of desired users are extracted from the code-multiplexed signals by despreading. It should be noted that in actuality, the channel identification codes (Gold codes) are multiplied before the Walsh codes.
Combiners 281 to 28M each add the N-number of results of multiplication that are output from respective ones of the multipliers 271 to 27M, thereby creating parallel data comprising M-number of symbols. A parallel/serial converter 29 converts this parallel data to serial data, and a data demodulator 30 demodulates the transmit data.
FIG. 22 is a diagram for describing the operation of the channel estimation unit. Here four pilot symbols (four OFDM pilot symbols) are multiplexed upon being dispersed within one frame composed of 32 OFDM symbols. Since one pilot symbol is composed of subcarrier samples equivalent to the number of subcarriers (M×N, e.g., 1024), subcarrier-by-subcarrier channel (amplitude characteristic and phase characteristic) estimation becomes possible by monitoring the FFT output at the pilot-receive timing on the receiving side. More specifically, to perform channel estimation, four sets of eight subcarrier samples in the frequency direction are gathered in the time direction to construct one group by a total of 32 subcarrier samples, as indicated at PG1 in FIG. 22, the average value of FFT outputs in this group is adopted as the channel value (amplitude and phase of the receive pilot signal) of the subcarrier at the center, and this channel value is compared with the known channel value (known amplitude and phase of the pilot signal), whereby the channel of this subcarrier is estimated. To obtain the channel estimation value of the next subcarrier, four sets of eight subcarrier samples shifted by one subcarrier in the frequency direction are gathered in the time direction to construct one group by a total of 32 subcarrier samples, as indicated at PG2, and the channel estimation value is similarly calculated using the average value in the group PG2. The reason for obtaining the channel value by averaging as set forth above is that since each symbol contains noise, the effects of such noise are eliminated by averaging to improve the S/N ratio. If subcarriers are very close in terms of frequency, the channel values are almost the same and therefore no problems are caused by averaging.
FIG. 23 is a diagram useful in describing FFT window timing in case of two paths (two waves), where A represents a direct wave and B a delayed wave (reflected wave). If the FFT window is decided to have a width Wa of one OFDM measured from the leading end of an OFDM symbol D1 of the direct wave, then the OFDM symbol D1 of the direct wave will merely overlap a guard interval GI1 and part of the D1 symbol of the delayed wave in the FFT window width Wa. As a result, there are no effects from inter-symbol interference (ISI) ascribable to multipath delayed waves. However, if the FFT window is decided to have a width Wb of one OFDM measured from the leading end of an OFDM symbol D1 of the delayed wave, then there will be overlap between part of the D1 symbol of the delayed wave and a guard interval GI2 of a D2 symbol of the direct wave, as a result of which the effects of inter-symbol interference ISI are sustained. The FFT window should be decided in such a manner that there will be no effects from inter-symbol interference ISI. If the FFT window is decided so as to be Wa in FIG. 23, inter-symbol interference ISI will not be received unless the largest delayed wave is delayed in excess of the duration of the guard interval GI.
If a Fourier transform of function ƒ(t) is expressed by F[ƒ(t)], a Fourier transform of a time-delay function ƒ(t−t0) will be exp(−2πjft0)F[ƒ(t)]. If we consider that exp(−2πjft0)=cos 2πft0−j sin 2πft0 holds, then the Fourier transform of the time-delay function ƒ(t−t0) will rotate in accordance with a change in frequency. For example, a Fourier transform of an impulse δ(t) at time t=0 is 1 (=a constant) at any frequency, as shown in (a) of FIG. 24. However, a Fourier transform of δ(t−t0) rotates in accordance with frequency in such a manner that the projection upon the I-jQ complex plane describes a unit circle, as shown in (b) of FIG. 24.
Thus, in a case where the receive power of a direct wave is high and that of a delayed wave is low, as shown in FIG. 25, the average value of the FFT becomes large and the amplitude of rotation small if the FFT operation is preformed upon deciding the FFT window using the OFDM symbol of the direct wave as a reference. Thus, since the fluctuation portion is small, a correct channel value can be obtained and the channel estimated correctly by averaging 32 subcarrier samples. In addition, there is no inter-symbol interference ISI.
However, in a case where the receive power of a direct wave is low and that of a delayed wave is high, as shown in FIG. 26, the average value of the FFT becomes small and the amplitude of rotation large if the FFT operation is preformed upon deciding the FFT window using the OFDM symbol of the direct wave as a reference. Thus, when the fluctuation portion becomes large, the channel value obtained by averaging 32 subcarrier samples includes an error and the channel cannot be estimated correctly.